The present invention is concerned with a denture-base resin composition and a method for using it.
As is well-known, dentures having a metal base is more tough, yet less breakable as compared with those composed exclusively of methyl methacrylate resin. The metalic denture can then be made so thin that the resulting denture produces a more natural "feel" to the patient while in service, is free from any water absorption properties, and is kept highly dimensional stable in use. In addition, the denture of the above-mentioned type has numerical advantages; its base has no or little irritating effect; it is compatible to the mouth tissues to the extent that it will not become unsanitary; it shows good thermal conductivity without sacrificing a kind of sensitivity to temperature or taste; it has sufficient strength to resist all normal masticating stresses; and the like. Thus, the denture of this type has spread with the years.
At the present time, metal materials that are used with any degree of success for denture-base materials include gold alloys, gold/silver/palladium alloys, platinum alloys, cobalt/chromium alloys, nickel/chromium alloys, 18-8 stainless-steel wrought alloys etc. Among these alloys, the cobalt/chromium or nickel/chromium alloys are becoming widespread since they are light in weight, say, about one half of the density of the gold alloys, and are very strong, say, about twice the Young's modulus of elasticity. In addition, they are inexpensive and excel in the resistance to discoloration. Artificial teeth used with these metal bases include porcelaineous and resinous teeth; however, preference is generally given to the porcelaineous teeth in view of hardness, wear resistance, harmlessness etc.
In some cases, the metal base may be formed as an integral piece from a metal. In most of the dentures having a metal base that are now available, an alveolus portion for fixing artificial teeth in place and a portion exposed to the inside of the cheek within the mouth are composed of a resin while a portion on the palate side and a connecting part are formed of a metal. Thus, advantage is taken of both metal and resin.
A methyl methacrylate resin is now widely used as a denture-base resin material. Indeed, however, this methyl methacrylate resin shows no chemical adhesion to dental metals or porcelaineous teeth at all. Although various proposals have been made to establish a mechanical connection between the resin and the metal or porcelaineous teeth, there is still left much to be desired. Typically, when the denture receives external forces such as masticating stresses in the mouth during use, the junction between the metal and the resin so-called finishing line cracks to cause separation of one from the other, thus resulting in discoloration and contamination of the resin, emission of an offensive odor and the like.
The necessity of maintenance of such a mechanical connection inevitably renders the denture design and the casting work complicated and difficult. In some cases, polishing of dentures may cause breakage of a thin portion of the junction between it and the metal. Use of retention beads in the junction imposes a certain limitation upon the color tone of the denture. As there is no choice but to cover the muscous surface of the denture base with a metal, it is impossible to effect lining of resin, often referred to as the so-called relining, even when adaptability of the base with the metal becomes unfit.
When dentures are fabricated from porcelaineous teeth and conventional denture-base resin, reliance is put upon either retainer means comprising gold pins set in the surface of the denture base or mechanical retention means using holes, since the porcelaineous teeth do not show any chemical adhesion to the base resin at all. This often causes breakage of the procelaineous teeth due to the concentration of stresses to part thereof held by said retainer means, this resulting in falling-off of the procelaineous teeth from a given place in the mouth during use, as reported in literature.
In addition, accumulation of the mouth fluids and occurrence of bacteria take place in the crevices in the interface of the resin base and the porcelaineous teeth during use, leading to discoloration of the denture and emission of an offensive odor.
With a view to eliminating the above-mentioned defects, it has been attempted to make a chemical bond between the porcelaineous teeth or metal and the base resin material.
Typically, it has been reported that the surface of a metal is etched with an inorganic acid, coated with adhesives and laminated with a methacryl resin. Alternative process using 4-methacryloxyethyl trimellitic acid have also been reported. However, these processes often offer several problems; they result in considerable drops in efficiency and marked variations in the adhesion to the cobalt/chromium alloys and the nickel/chromium alloys. In these processes, any adhesion to the porcelaineous teeth is not obtained at all.
In order to bond the base resin to the porcelaineous teeth, it has been reported to incorporate a silane compound into the methyl methacrylate. With this process, it is possible to bond the base to the porcelaineous teeth, but it is difficult to bond the base to the metal. There are also considerable variations in the adhesive force.
On the other hand, there is well-known in the art such a composition that shows adhesion only to the cobalt or nickel/chromium alloys or the porcelaineous teeth; however, such a composition that is simultaneously displays strong chemical adhesion to both the cobalt or nickel/chromium alloys and the porcelaineous teeth is not still revealed in public.